I delegate to methods of self with the form 'render_METHOD' where METHOD
is the HTTP that was used to make the request. Examples: render_GET,
render_HEAD, render_POST, and so on. Generally you should implement those
methods instead of overriding this one.

render_METHOD methods are expected to return a string which will be the
rendered page, unless the return value is twisted.web.server.NOT_DONE_YET,
in which case it is this class's responsibility to write the results to
request.write(data), then call request.finish().

Old code that overrides render() directly is likewise expected to return
a string or NOT_DONE_YET.